A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning structure, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
There is a desire to integrate an ever-increasing number of electronic components in an IC. To achieve this it is necessary to decrease the size of the components and therefore to increase the resolution of the projection system which images the pattern of the mask onto the substrate. Increasing the resolution of the projection system can enable increasingly smaller details, or line widths, to be projected on a target portion of the substrate. This means that the projection system and the lens elements used in the projection system must comply with very stringent quality requirements. Despite the great care taken during the manufacturing of lens elements and the projection system, the projection system may still suffer from wave front aberrations, such as, for example, displacement, defocus, astigmatism, coma and spherical aberration. Such aberrations are important sources of variations of the imaged line widths occurring across the image field. It is important that the imaged line width at different points within the image field are substantially constant. If the line width variation is large, the substrate on which the image field is projected may be rejected during a quality inspection of the substrate. Using techniques such as phase-shifting masks or off-axis rumination, the influence of wave front aberrations on the imaged line-widths may further increase.
Therefore, it is important to be able to measure the wave front aberrations of the projection system accurately to ensure that the stringent imaging quality requirements are satisfied or, if necessary, to control the reduction of the aberration (for example the position of certain lens elements in the projection system may be adjusted in order to minimize wave front aberrations). Some techniques are known for measuring wave front aberrations, as will be described in more detail below; these techniques can suffer from systematic errors and hence limitation in accuracy. They also suffer from the problem of strict tolerance in the positioning of certain elements when imaging test patterns which can be difficult and/or expensive.